As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries as an energy source has also sharply increased.
Depending upon kinds of external devices in which secondary batteries are used, the secondary batteries may be used in the form of a single battery or in the form of a battery pack having a plurality of unit cells electrically connected to each other. For example, small-sized devices, such as mobile phones, can be operated for a predetermined period of time with the power and the capacity of one battery. On the other hand, a battery pack including a plurality of batteries needs to be used in middle or large-sized devices, such as laptop computers, portable digital versatile disc (DVD) players, small-sized personal computers (PCs), electric vehicles, and hybrid electric vehicles, because high power and large capacity are necessary for the middle or large-sized devices.
Among secondary batteries, a lithium secondary battery is widely used since the lithium secondary battery has high power and large capacity.
However, various kinds of combustible materials are contained in the lithium secondary battery. As a result, the lithium secondary battery may be heated or explode due to overcharge of the lithium secondary battery, overcurrent in the lithium secondary battery, or other external physical impact applied to the lithium secondary battery. That is, the safety of the lithium secondary battery is very low. Consequently, safety elements, such as a positive temperature coefficient (PTC) element and a protection circuit module (PCM), to effectively control an abnormal state of the lithium secondary battery, such as overcharge of the lithium secondary battery or overcurrent in the lithium secondary battery, are loaded on a battery cell in a state in which the safety elements are connected to the battery cell.
As described above, one or more battery cells are used in each of the small-sized devices, whereas a middle or large-sized battery module including a plurality of battery cells electrically connected to each other is used in each of the middle or large-sized devices, such as vehicles, because high power and large capacity are necessary for the middle or large-sized devices. Size and weight of a battery module are directly related to a battery module installation space and power of a corresponding middle or large-sized device. For this reasons, manufacturers are trying to manufacture small and lightweight battery modules.
Generally, a plurality of unit cells is mounted in a cartridge in a state in which the unit cells are connected in series or parallel to each other, and a plurality of cartridges is electrically connected to each other to manufacture a battery pack.
FIG. 1 is a view typically showing series connection between unit cells of a conventional high-power, large-capacity battery pack.
Referring to FIG. 1, each unit cell is configured to have a structure in which a cathode, an anode, and a separator are provided in a case together with an electrolyte in a sealed state and a cathode tab 20 and an anode tab 30 protrude from upper and lower ends of the case. In a case in which a first unit cell 10 is disposed such that a cathode tab 20 is located at the upper end thereof, a second unit cell 11 disposed adjacent to the first unit cell 10 is disposed such that a cathode tab 21 is located at the lower end thereof. Opposite electrodes of the unit cells 10 and 11 are electrically connected to each other via an electrode lead 40 in a state in which the opposite electrodes of the unit cells 10 and 11 are adjacent to each other. A third unit cell 12 is connected in series to the second unit cell 11 in the same manner as connection between the first unit cell 10 and the second unit cell 11. Although not shown in FIG. 1, a first battery group of FIG. 1 including a plurality of unit cells connected in series to each other as described above is connected in parallel to a second battery group including a plurality of unit cells connected in the same manner as in the first battery group.
The battery groups, which are connected in parallel to each other as described above, are mounted in a housing 70 in a state in which the cathode tab 20 of the first unit cell 10 of each of the battery groups is connected to a cathode external terminal 50 and an anode tab 33 of a last unit cell 15 of each of the battery groups is connected to an anode external terminal 60. As shown in FIG. 1, the electrode tabs 20 and 30 are formed in the major axis direction of the unit cell 10. According to circumstances, the electrode tabs 20 and 30 may be formed in the minor axis direction of the unit cell 10.
In the battery pack including the unit cells 10, 11, 12, 13, 14, and 15, which are electrically connected to each other, however, the unit cells 10, 11, 12, 13, 14, and 15 have the same size or capacity. In order to reduce weight and thickness of the battery pack in consideration of design of a device to which the battery pack is applied, therefore, it is necessary to reduce the capacity of the battery pack. Alternatively, it may be necessary to change the design of the device such that the size of the device can be reduced. During a design changing process, however, electrical connection between the unit cells is complicated with the result that it may be difficult to manufacture a battery pack satisfying desired conditions.
Consequently, there is a high necessity for a high-power, large-capacity battery pack including a plurality of connected unit cells wherein the battery pack can be configured based on the form of a device to which the battery pack is applied while the capacity of the battery pack is maintained.